Yeah two reasons for distilled water, both related to the impurities in it.

The main one is corrosion, and the second surprisingly is conductivity. Pure distilled water is apparently a lot less likely to cause a full short for a small leak, it's the impurities that conduct. That's why distilled water cooling should be changed every year or so. It picks up impurities from the block and pump over time.

The beer... well, not sure how many folks in Canada haven't seen the gunk in the bottom of a beer bottle after it's been in the case "almost empty" for a couple of weeks..... :)